A typical shift mechanism of an automatic transmission utilizes a combination of a plurality of planetary gearsets. A powertrain of such an automatic transmission that includes the plurality of planetary gearsets changes rotation speed and torque received from a torque converter of the automatic transmission, and accordingly changes and transmits the changed torque to an output shaft.
It is well known that when a transmission realizes a greater number of shift speeds, speed ratios of the transmission can be more optimally designed and therefore a vehicle can have better fuel mileage and better performance. For that reason, an automatic transmission that enables more shift speeds is constantly under investigation.
In addition, with the same number of speeds, the features of a powertrain, such as durability, efficiency in power transmission, and size depend a lot on the layout of the combined planetary gearsets. Therefore, designs for combined structure of a powertrain are also constantly under investigation.
Furthermore, a manual transmission that has too many speeds is inconvenient to use as it requires a driver to shift frequently. Therefore, the positive features of having more shift-speeds are more relevant and appropriate for automatic transmissions, because automatic transmissions automatically control shifting operations without needing manual operation.
In addition to various developments regarding four and five speed powertrains, powertrains of automatic transmissions realizing six forward speeds and one reverse speed have recently been introduced.
These six-speed powertrains may include multiple friction elements, clutches and brakes. However, the more friction elements used the heavier and larger the automatic transmission.
Such automatic transmissions also have durability problems when their elements always rotate at high speeds.
Therefore, an automatic transmission that addresses the above drawbacks while improving efficiency of power transmission would be highly desirable.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known in this country to a person of ordinary skill in the art.